I think you should skip the VPS when considering your upgrade path. When you're on the shared server, the other sites competing for the server resources are rather static, so your vBulletin should have enough breathing room for quite some time. If you upgrade to a VPS you will be competing for resources with some very active sites, many of which use the MySQL server and that's where vBulletin might choke. Best to eventually skip over the VPS and go for semi- or dedicated hosting.